Arsenic associated with mineral matrices seldom poses a direct environmental risk, whereas arsenic that is mobilized in the aqueous phase poses a potential threat to human and environmental health. Consequently, controlling arsenic's sequestration by solids also controls its associated risk. Chemical reactions of arsenic occurring at the solid-water interface (including adsorption and desorption, precipitation and dissolution, and reduction and oxidation) not only govern the release of arsenic into water, but form the basis of arsenic removal technologies. Thus, the enhanced fundamental understanding of arsenic behavior at critical solid-water interfaces that this project expects to achieve can be applied to both prevention and remediation of arsenic contamination. Iron-based solids are typically used to remove arsenic from contaminated water and are the typical solids with which arsenic is associated in natural aerobic environments. However, our current work has shown they are unstable when placed in the anaerobic environments that typify many arsenic-bearing waste disposal sites. The reverse is true for arsenic associated with sulfides, such as at mine impacted sites, where the shift from anaerobic to aerobic environments stimulates arsenic release. Thus, the behavior of minerals containing iron and sulfide when subjected to changing redox environments is the primary focus of the proposed work. The project's specific aims are to determine the mechanisms and pathways for 1) arsenic association with iron solids and 2) arsenic association with sulfur solids, and to develop 3) engineered intervention approaches that utilize biological and biogeochemical mineral retention processes to minimize arsenic release from solid wastes. These solid-arsenic-water reactions of interest are typically microbially mediated and may take multiple pathways and lead to multiple final solid phases with varying capacity for arsenic retention. Because of the complexity of the relevant processes, the project includes experts in aqueous geochemistry, microbiology, chemical dynamic modeling, process engineering and spectroscopy.